1. Field of the Invention
The present invention relates generally to the art of automatic transmissions for vehicles but more particularly rather to controlling an internal combustion engine in a vehicle in relation to control of the automatic transmission.
2. Description of the Prior Art
It is already known to control the vehicle's internal combustion engine in relation to the speed stage shifting control of the automatic transmission in various ways such that the engine output torque is temporarily lowered during the speed stage shifting of the automatic transmission for the purpose of improving smoothness of speed stage shifting of the automatic transmission, as described in Japanese Patent Laying-open Publications 61-112850, 61-150837, 61-253229, 62-265039 and 62-279144 and U.S. Pat. Nos. 4,266,447, 4,355,550, 4,370,903, 4,403,527 and 4,819,187. An essential matter in the art of temporarily decreasing the engine output torque during the speed stage shifting of the automatic transmission in which a certain first selective torque transmitting means, such as a clutch or a brake in the automatic transmission, is disengaged while a certain second selectively torque transmitting means is newly engaged, is the timing of decreasing and regaining the engine output torque in relation to the engagement of said second selectively torque transmitting means. If the timing of decreasing the enging output torque is too late in this respect, the effect of temporarily decreasing the engine output torque is not well exhibited, whereas if the timing of regaining the engine output torque is too early, the effect of temporarily decreasing the engine output torque is much reduced.
Taking a downshifting from a certain higher speed stage to a certain lower speed stage for an example of the speed stage shifting of the automatic transmission, the input rotational speed of the transmission gradually increases from a first rotational speed corresponding to the gear ratio of said certain higher speed stage, starting at a time point at which the selective torque transmitting means which had been engaged to provide said certain higher speed stage was virtually disengaged, to a second rotational speed corresponding to the gear ratio of said certain lower speed stage provided that the vehicle continues to run at a substantially constant speed. Therefore, it is theoretically possible to obtain the time point at which the selective torque transmitting means to set up said certain lower speed stage is virtually engaged by monitoring the input rotational speed of the transmission by means of a certain conventional speed sensor. For the convenience of clarity of analysis, if the selective torque transmitting means which is engaged to set up said certain lower speed stage is a one way clutch, such a time point at which the selective torque transmitting means for said certain lower speed stage is virtually engaged will be more distinctly conceived as a time point at which the input rotational speed of the transmission has just increased as much as a ratio corresponding to the ratio of the reduction gear ratio of said certain lower speed stage to the reduction gear ratio of said certain higher speed stage. However, even in the case of a one way clutch, it is not possible, as a matter of practice, to obtain the time point at which the one way clutch is engaged immediately at the instant of engagement by a monitoring of the input rotational speed of the transmission, because even as a matter of logic a certain time lapse is required to confirm that said rotational speed which had been gradually increasing has now stopped increasing.
On the other hand, a convenient method of temporarily decreasing the engine output torque is, in the case of a gasoline engine, to delay the spark timing. According to this method, the engine output torque can be decreased at a relatively high level of responsiveness. However, when the engine operation is controlled by a microcomputer system as in many modern automobiles, since the central processing unit must carry out various computations in succession, there is a certain time lapse before the spark timing is virtually delayed from the time point at which the spark timing delay is instructed to the microcomputer engine control system.
Because of the above situation it was a conventional art of temporarily decreasing the engine output torque during the speed stage shifting of the automatic transmission to monitor the input rotational speed of the transmission so as to obtain a time point at which the input rotational speed of the transmission traverses a certain predetermined value which is as much as a certain difference in advance of the final input rotational speed of the transmission expected to be attained when the speed stage shifting has been completed, to generate a signal for instructing the microcomputer engine control system of delaying the spark timing as much as an amount which has been calculated in the meantime after the speed stage shifting was started, then to maintain the delay of spark timing for a certain determinate period, and then to cancel the delay of spark timing to regain the engine output torque.
Such an art is illustrated in FIG. 1 of the accompanying drawings, in which on and off of spark timing delay demand signal, carrying out of the spark timing delay with its calculated amount, input rotational speed of the transmission, output torque of the transmission, and hydraulic pressure for actuating the selective torque transmitting means to be disengaged for the speed stage shifting are shown in relation to a common time scale with respect to an example of downshifting from a certain higher speed stage to a certain lower speed stage in which a one way clutch is automatically engaged in exchange with disengagement of a hydraulically operated selective torque transmitting means.
Referring to FIG. 1, as the hydraulic pressure in the selective torque transmitting means is exhausted as shown by a solid line with an accompanying decrease of the output torque of the transmission also shown by a solid line, at time point t.sub.1 the input rotational speed of the transmission starts to increase as shown by a solid line. According to this process, it is theoretically expected that at time point t.sub.2 the one way clutch engages and the rotational speed of the input shaft stops increasing. According to the conventional method, the input rotational speed of the transmission is monitored to obtain a time point t.sub.a corresponding to a certain input rotational speed of the transmission which is as much as a difference Ia in rotational speed in advance of the input rotational speed of the transmission which is expected to be attained when the speed stage shifting was completed, provided that the vehicle continues to run at a constant speed for the time being. When the time point t.sub.a was obtained, a signal for instructing the microcomputer engine control system to delay the spark timing is generated, and after a certain time delay T.sub.1 the delay in spark timing is virtually started to be maintained for a certain period T.sub.2, and then the amount of spark timing delay is gradually decreased over time period T.sub.3.
It is possible to obtain the time point t.sub.a instantly when the input rotational speed of the transmission monitored by a rotational speed sensor traversed the corresponding certain value. Therefore, by appropriately determining the amounts of I.sub.a, T.sub.1, T.sub.2 and T.sub.3, although the amount of T.sub.1 cannot be smaller than a certain minimum amount, a high quality speed stage shifting of the automatic transmission would be available as supported by a temporary decrease of engine output torque at good timing.
However, it is in fact unavoidable that the exhausting of hydraulic pressure from the selective torque transmitting means fluctuates as shown by a broken line or a dot dash line in FIG. 1 according to hydraulic fluid temperature, manufacturing allowance of the hydraulic system, etc. If the hydraulic pressure is exhausted along the broken line, the input rotational speed of the transmission will increase along a broken line therefor, whereas if the hydraulic pressure is exhausted along the dot dash line, the input rotational speed of the transmission will increase along a dot dash line therefor. When the input rotational speed of the transmission increases along the broken line, the time duration Ta for advancing the time point t.sub.a relative to the time point t.sub.2 is shortened to Ta', whereas if the rotational speed of the transmission increases along the dot dash line the time duration Ta is elongated to Ta". The difference between Ta and Ta' or between Ta and Ta" is greater as the advancing difference in rotational speed Ia is greater. However, a certain substantial amount is required for Ia in order to ensure a certain required minimum amount for T.sub.1 as described above. When the spark timing delay demand signal is dispatched later than required relative to the time point of completion of the speed stage shifting, i.e., the engagement of the one clutch according to the present example, due to the shortening of the time duration Ta to Ta', the temporary decrease of the engine output torque occurs too late relative to the completion of the speed stage shifting, i.e. the engagement of the one way clutch in the present example, and therefore the output torque of the transmission will fluctuate as shown by a broken line in the diagram therefor, whereas when the spark timing delay demand signal is dispatched earlier than desired due to the elongation of Ta to Ta", the temporary decrease of the engine output torque will end too soon, and therefore the output torque will fluctuate as shown by the dot dash line in the diagram.